The invention relates to the field of liquid crystal display technology, and more particularly, to a pixel driving circuit and a display device.
Currently, display devices with active-matrix organic light emitting diode (AMOLED) panels, which consist of columns and rows of AMOLED pixels, are widely used in a variety of products. For AMOLED pixels, a pixel driving circuit constructed in thin film transistors (TFTs) supplies corresponding currents to organic light-emitting diodes (OLEDs). In a case of a basic pixel driving circuit for AMOLED, as shown in FIG. 1, is specifically a 7T1C circuit that includes seven transistors and a capacitor.
FIG. 1 is equivalent to a conventional 7T1C pixel driving circuit, and FIG. 2 shows a driving time sequence diagram. The working principle of the 7T1C pixel driving circuit is as follows: in a preparation stage t1, a second scan signal scan[n−1] is at a low voltage level, and thus a fourth transistor T4 turns on, a potential of a reference point A becomes low, and a first capacitor C1 is proceeding with charge; in a compensation stage t2 of a threshold voltage Vth of a first transistor T1, a first scan signal scan[n] is at a low voltage level, and thus a second transistor T2, a third transistor T3, and a seventh transistor T7 turn on. Owing to a negative voltage applied on a gate electrode of the first transistor T1, a source electrode and a drain electrode of the first transistor T1 form a short circuit, and the potential of the reference point A follows the relation expression: |VA|>|Vth|. That is, by this time, the first transistor T1 becomes a diode and turns on, the reference point A is charged by a voltage signal of grayscale data Vdata through the first transistor T1 until the potential of the first reference point A turns into be equal to: Vdata−|Vth| while the first transistor T1 is in a cut-off state, and moreover a light emitting device OLED is restored because of turning on the seventh transistor T7; in a displaying stage t3, an enabling signal EM is at a low voltage level, and a fifth transistor T5 and a sixth transistor T6 turn on, wherein a gate-to-source voltage of the first transistor T1 is calculated by: Vgs=Vdd−(Vdata−|Vth|), and a current, which passes through the source electrode and the drain electrode of the first transistor T1 and then passes through the light emitting device OLED, is calculated according to the formula:
            I              d        ⁢        s        ⁢        1              =                            (                      1            2                    )                ⁢                              K            ⁡                          [                                                V                                      d                    ⁢                    d                                                  -                                  (                                                            V                      data                                        -                                                                                        V                                                  t                          ⁢                          h                                                                                                                            )                                -                                                                        V                                          t                      ⁢                      h                                                                                                    ]                                2                    =                        (                      1            2                    )                ⁢                              K            ⁡                          (                                                V                                      d                    ⁢                    d                                                  -                                  V                  data                                            )                                2                      ,where K=CoxμW/L. The light emitting device OLED works in the displaying stage t3. For example, a display panel with resolution of 1440*2960/18.5:9. Scanning frequency of the pixel driving circuit is 60 hertz, that is, a gate driving time t1 or a gate driving time t2 is equal to: 1/60/(1440+blank), and is approximately 6 microseconds, wherein the blank, an amount of displacement, can be ignored. Because a frame of time is 1/60 seconds and is equal to: t1+t2+t3, a driving time, calculated by: t3= 1/60−t1−t2, is 16.7 milliseconds. However, for the TFTs that turn on in the stage t3, the driving time is very long. The reasons why the first transistor T1, the fifth transistor T5 and the sixth transistor T6 turn on are that the enabling signal EM is at a low voltage level for a long time and the gate electrode of the first transistor T1 is also at a low voltage level for a long time in a light emitting stage due to the first capacitor C1, causing the first transistor T1, the fifth transistor T5, and the sixth transistor T6 to be in an on state for a long time.
Because TFTs are in an on state for a long time, TFT devices are in a bias stress stage for a long time, causing electric characteristics of the devices, such as a turn-on voltage and an electron mobility, to drift. Thus, the display performance of the whole screen is affected, and lifespan of the TFT devices reduce.
In view of this, a kind of solution is urgently required to solve the foregoing problems.